1. Field of the Invention
The present invention relates to a process for the preparation of crystalline titanium-containing molecular sieves of the composition (SiO.sub.2).sub.1-X (TiO.sub.2)x, (0,0001.ltoreq.x.ltoreq.0,03) from a tetraalkyl orthosilicate, a tetraalkyl orthotitanate and an aqueous template solution.
2. Prior Art
Crystalline molecular sieves of the composition (SiO.sub.2).sub.1-X (TiO.sub.2).sub.x are known in which titanium atoms replace some of the silicon atoms in the crystal lattice. These are specifically titanium silicalite-l of the MFI crystal structure (DE 30 47 798), titanium silicalite-2 of the MEL crystal structure (BE 1 001 038 and J. S. Reddy, R. Kumar, P. Ratnasamy, Appl. Catal. 58 (1990) L1, titanium beta-zeolite of the BEA crystal structure (M. A. Camblor, A. Corma, A. Martines, I. Perez-Pariente, J. Chem. Soc., Chem. Commun. 1992, 589 and EP 659 685) and TS-48 with the crystal structure of zeolite ZSM48 (D. P. Serrano, H. X. Li, M. E. Davis, J. Chem. Soc., Chem. Commun. 1992, 745).
The known crystalline titanium-containing molecular sieves are used as catalysts, especially for oxidation reactions with hydrogen peroxide or organic hydroperoxides. Examples are the use of titanium silicalite-1 as a catalyst for the reaction of olefins with hydrogen peroxide to give epoxides (EP 100 119), the reaction of aromatics with hydrogen peroxide to give hydroxyaromatics (DE 31 35 559), the reaction of aliphatic hydrocarbons with hydrogen peroxide to give alcohols and ketones (EP 376 453) and the reaction of cyclohexanone with hydrogen peroxide and ammonia to give cyclohexanone oxime (EP 208 311). Examples for the use of titanium silicalite-2 are the hydroxylation of phenol (J. S. Reddy, S. Sivasanker, P. Ratnasamy, J. Mol. Catal. 71 (1992) 373) and the reaction of cyclohexanone with hydrogen peroxide and ammonia to give cyclohexanone oxime (J. S. Reddy, S. Sivasanker, P. Ratnasamy, J. Mol. Catal. 71 (1992) 383). Titanium beta-zeolite can be used as a catalyst for the reaction of olefins with hydrogen peroxide or organic hydroperoxides to give epoxides (A. Corma, P. Steve, A. Martines, S. Valencia, J. Catal. 152 (1995) 18 and EP 659 685).
The known crystalline titanium-containing molecular sieves are prepared by hydrothermal synthesis. In the first step a tetraalkyl orthosilicate as a silicon source and a tetraalkyl orthotitanate as a titanium source are hydrolyzed with a tetrasubstituted ammonium hydroxide as a basic and structure-determining component (template), in the presence of water, the alcohol formed from the hydrolysis of the tetraalkyl orthosilicate and the tetraalkyl orthotitanate is then distilled off, the resulting sol is crystallized at a temperature above 100.degree. C. under pressure and the solid formed is separated off, washed, dried and calcined at a temperature above 300.degree. C.
It is further known to prepare titanium silicalite-1 from tetraethyl orthosilicate and tetraethyl orthotitanate. Here the synthetic sol is prepared by hydrolyzing a mixture of tetraethyl orthosilicate and tetraethyl orthotitanate by the addition of aqueous tetrapropylammonium hydroxide solution. The alcohol formed is then distilled off and the resulting sol is crystallized at a temperature of 175.degree. C. under autogenous pressure (U.S. Pat. No. 4,410,501).
It is known from EP 543 247 that the tetrapropylammonium hydroxide can be replaced with a combination of tetra-npropylammonium bromide and ammonia in the synthesis of titanium silicalite-1.
Titanium silicalite-2 (MEL structure) and titanium betazeolite (BEA structure) are prepared using tetra-nbutylammonium hydroxide and tetraethylammonium hydroxide, respectively.
According to B. Notari, Stud. Surf. Sci. Catal. 67 (1991) 243, the hydrolysis of the mixture of the silicon and titanium components with the aqueous solution of the basic template is a critical step which is liable to cause problems. Thus, the formation of a titanium- containing precipitate in the preparation of the sol must be avoided under all circumstances; otherwise the titanium is not available for the crystallization and materials of lower catalytic activity, which are moreover contaminated with titanium dioxide, are formed.
According to A. J. H. P. van der Pol et al., Appl. Catal. A. 92 (1992) 93-111, the formation of a sparingly soluble precipitate in the preparation of a synthetic sol for the synthesis of titanium silicalite can be avoided if the mixture of tetraethyl orthosilicate and tetraethyl orthotitanate is first cooled to 0.degree. C. and the aqueous tetrapropylammonium hydroxide solution, also precooled to 0.degree. C., is metered in dropwise, with stirring. The same applies to the synthesis of titanium silicalite-2 and titanium beta-zeolite.
The known processes have the disadvantage that the preparation of the synthetic sol is expensive and liable to cause problems. Thus, the metering of the aqueous template solution takes several hours and the temperature and rate of addition have to be precisely maintained. Also, an additional working step is required for the distillation of the alcohol formed in the hydrolysis.